The present invention is directed to a method of non-ferrous casting using a tool steel, and casting components made from the tool steel, and in particular, to a tool steel casting mold that is extremely corrosion and seizure resistant when used in methods of non-ferrous metal casting.
When casting non-ferrous metals or alloys containing aluminum, magnesium, or zinc, consideration must be given to the adverse effects of corrosion and seizure on the components used during the casting process. To combat these effects, tool steels are used for the dies and the structural parts of the casting machines, injection molding machines, hot forging machines, and extrusion machines. Even so, when casting an aluminum alloy, the tool steel casting components, e.g., the molds, dies, cores, insert pins, supply pipes, gates, and the like of the casting apparatus, can prematurely corrode due to contact with the molten aluminum alloy. The corrosion can take the form of galling or seizing of the component. Such corrosion can then cause defects in the cast product, e.g., convex-type defects, and these defects can make it difficult to remove the cast product from the mold.
When casting non-ferrous alloys such as aluminum alloys, mold casting is often used. Mold casting can involve a number of different techniques such as permanent mold casting, low-pressure permanent mold casting, die casting, and squeeze casting. The type of mold casting used is dependent upon factors such as the shape and size of the article being manufactured, the required dimensional accuracy, the number of articles to be manufactured, the required quality, the required mechanical properties, and cost considerations.
Each of casting techniques noted above utilizes a different procedure to shape the molten non-ferrous metal. Permanent mold casting involves introducing a molten metal into a mold under the force of gravity. Low-pressure permanent mold casting applies a pressure to the surface of a molten metal, e.g., on the order of 0.01 to 0.03 MPa. The molten metal is then forced upward into a mold and against the force of gravity to fill the mold.
Die casting methods pour molten metal into a mold with the molten metal being under a pressure of about 40 to 100 MPa, or under gravity conditions.
Squeeze casting first introduces a molten metal into a mold in the absence of air. Then, a pressure of 50 to 120 MPa is applied and the molten metal is solidified.
In mold casting, particularly, low-pressure permanent and permanent mold casting, a mold coating is applied to the surface of the mold to protect the mold surface from the molten non-ferrous metal alloy. Typically, a mold coating is applied over the mold surface prior to casting as a means to facilitate cast product removal and to protect the mold. One example of prior art mold coatings generally comprises, in weight percent, about 40-50% of liquid glass, about 45-55% MgO, and about 5-10% water.
Each of the molds associated with these casting techniques suffers from some type of corrosion or other effect, which reduces the mold life span. In die casting, the molds can exhibit heat checking, cracking and erosion. Permanent and low-pressure permanent mold casting molds are susceptible to corrosion, and molds for squeeze casting suffer from heat checking and cracking.
In the past, steels for the manufacture of molds have typically been hot-work tool steels having a chromium content of about 5% by weight. However, these alloys do not always provide satisfactory corrosion or softening resistance, even with mold coatings. As such, prior art solutions have been proposed to overcome this problem.
One prior art solution to the corrosive effects of molten non-ferrous metals or alloys such as aluminum alloys is to surface treat the tool steel component by nitrocarburizing, and form a protective layer on the component. The problem with this solution is that the protective layer is eroded over time, and the layer on the tool steel component is eventually worn away, thus permitting corrosion to occur.
Other solutions in the prior art have been proposed through adjustments in the tool steel alloy composition. Japanese Publication No. 11-279702 teaches that the resistance against aluminum corrosion of a die-cast mold can be improved by the intentional addition of a large content of sulfur to a steel alloy composition containing carbon, silicon, manganese, chromium, molybdenum, vanadium, and iron.
Japanese Publication No. 2000-144334 provides another solution in the way of alloy composition adjustment. This publication teaches the combined addition of S and Te to improve resistance to aluminum corrosion during die casting in a steel alloy containing carbon, silicon, manganese, chromium, molybdenum, vanadium, and iron.
While the addition of sulfur or sulfur and tellurium improve corrosion, the level of sulfides are increased and toughness is lowered.
Another alloy composition proposed to alleviate aluminum corrosion in die casting components is a 5% chromium steel composition designated as H13 under the specification of the American Society for Testing and Materials. ASTM H13 is described in Japanese Publication No. 11-152549 as an alloy useful under high temperature conditions. However, the life of this alloy can be shortened by its insufficient resistance to softening at high temperatures, and lack of adequate heat-check and corrosion resistance. Japanese Publication No. 11-152549 also discloses an alloy with improved performance over the ASTM H13 alloy by providing a tool steel alloy composition wherein the composition consists of, in weight percent, 0.10-0.50% carbon, not more than 0.5% silicon, not more than 1.5% manganese, not more than 1.5% nickel, between 3.0 and 13.0% chromium, 0-3.0% molybdenum, 1.0-8.0% tungsten, 0.01-1.0% vanadium, 0.01-1.0% niobium, 1.0-10.0% cobalt, 0.003-0.04% boron, 0.005-0.05% nitrogen, with the balance iron and unavoidable impurities. The improved alloy is superior to softening at high temperatures in comparison to the ASTM H13 alloy due to the presence of cobalt, but cobalt reduces toughness. Further, this alloy""s softening performance is still inadequate.
In spite of the advancements in tool steel alloy compositions, the presently available prior art tool steel alloys still suffer from inadequate resistance to molten aluminum alloy corrosion, excessive softening at high temperatures, and poor toughness. Accordingly, a need has developed to provide casting components that have increased resistance to corrosion and softening when exposed to non-ferrous casting conditions and better toughness.
The present invention solves this need through the discovery that a steel alloy intended for use in boiler tube construction unexpectedly provides superior performance when used as a casting component in methods of casting non-ferrous metals. Using this high chromium steel offers improved resistance to molten aluminum corrosion, resistance to degradation when the casting components are treated to remove unwanted material between casting sequences, and other benefits detailed below.
Boiler tube steel alloys are disclosed in U.S. Pat. Nos. 5,069,870 and 5,240,516 to Iseda et al., both hereby incorporated in their entirety by reference. However, neither of these patents teaches that the disclosed steels are suitable for use as a casting component in non-ferrous casting apparatus, nor do they recognize the benefits obtained when such steels are used to make casting components such as molds and used in non-ferrous casting methods.
Accordingly, it is a first object of the present invention to provide a tool steel which is ideally adapted for use as a casting component during the casting of non-ferrous metals.
Another object of the invention is a tool steel for use particularly in the casting of aluminum alloys into product shapes.
Yet another object of the invention is a method of casting non-ferrous metals wherein one or more components of the casting apparatus that come into contact with the molten non-ferrous metal comprises a steel containing carbon, manganese, silicon, phosphorous, sulfur, chromium, nickel, molybdenum, and vanadium, and optionally, cobalt, titanium, niobium, tungsten, copper, with the balance iron and inevitable impurities.
Yet another object of the invention is a tool steel for use as a mold in a non-ferrous casting method, particularly, methods that apply mold coatings to molds prior to casting and maintain the molds by shot blasting techniques after casting.
Other objects and advantages of the present invention will become apparent as a description thereof proceeds.
In satisfaction of the foregoing objects and advantages, the present invention provides an improvement in the components used in connection with the casting of non-ferrous metals. The invention, in one aspect, provides an improved casting component made from an alloy steel composition comprising, in weight percent, from about 0.05 to about 0.4% carbon; from about 0.10 to about 1.5 silicon; from about 0.1 to about 1.5% manganese; up to 2.0% nickel; from about 7.0 to about 15.0% chromium; up to 2.0% copper; up to 1.0% molybdenum; up to 3% tungsten; from about 0.05 to about 1.5% vanadium; up to 0.5% niobium; up to 0.1% aluminum; up to 0.1% nitrogen; up to 0.02% boron; up to 0.05% titanium; with the balance being iron and inevitable impurities. The casting component is one that comes into contact with the molten non-ferrous metal being cast and offers superb resistance to corrosion, oxidation, softening, checking, degradation, deformation, checking, and the like.
In a preferred embodiment, the steel composition includes, in weight percent, one or both of molybdenum from about 3 and 7% by weight and cobalt from about 1 to 10%. The casting component also has a chromium oxide layer adjacent a matrix formed of the steel composition of the casting component, the chromium oxide layer having a thickness ranging between about 1 and 30 microns. The chromium oxide layer is especially effective in providing the resistance to oxidation, corrosion, degradation, and deformation. The chromium layer can be the outermost layer of the component or be positioned between the matrix material and an iron oxide outer layer.
The casting component can be any type of a component used in casting of non-ferrous metals, including: a mold, a core, a sleeve, an insert for permanent mold casting; a stalk and mold for low-pressure permanent mold casting; and a plunger, a cylinder, a nozzle, a nozzle seat, a plunger tip, a ladle, a shot chamber, a tube, an ejection pin, a sprue spreader, and a ram for die casting.
The invention also entails the use of the casting component in a non-ferrous casting method wherein molten non-ferrous metal contacts one or more casting components and is cast into a desired shape. The casting method can be any known method used for casting of non-ferrous metals, but is preferably methods such as permanent mold casting, die casting, low-pressure permanent mold casting, and squeeze casting.
When employing a mold as the casting component, a portion of the mold designed to contact the molten non-ferrous metal can be coated with a mold coating as part of each casting sequence. When employing this mode of the invention, the mold coating contributes to formation of the chromium oxide layer during the casting operation and superior casting component performance.
While any non-ferrous metals can be employed as part of the inventive method, it is preferred to cast highly corrosive alloys such as aluminum-, zinc-, or magnesium-based alloys using the casting component composition noted above.
The casting component can be subjected to a metal removal process, e.g., shot blasting, between casting operations to prepare the surface for the next casting sequence. When utilizing a protective coating, the coating is then reapplied to the shot blast portion of the casting component for subsequent contact by the molten non-ferrous metal.
In yet another aspect of the invention, at least the corrosion resistance of casting molds is improved by forming the casting component as a mold, coating the mold with a mold coating, and forming the chromium oxide layer as part of the casting process. Use of the alloy steel composition as a mold and formation of the chromium oxide layer contributes to enhanced mold performance, both from a corrosion and softening resistance standpoint, and better maintenance of mold dimensional accuracy in spite of continued mold maintenance steps such as shot blasting.